1. Field of the Invention
The present invention relates to a bubble-jet type inkjet printhead. More particularly, the present invention relates to an inkjet printhead having a hemispherical ink chamber and an anti-wetting film.
2. Description of the Related Art
In general, inkjet printheads are devices for printing a predetermined image by ejecting small droplets of printing ink to desired positions on a recording sheet. Ink ejection mechanisms of an inkjet printer are generally categorized into two different types: an electro-thermal transducer type (bubble-jet type), in which a heat source is employed to form bubbles in ink causing an ink droplet to be ejected, and an electromechanical transducer type, in which an ink droplet is ejected by a change in ink volume due to deformation of a piezoelectric element.
There are multiple factors and parameters to consider in making an inkjet printhead having a bubble-jet type ink ejector. First, it should be simple to manufacture, have a low manufacturing cost, and be capable of being mass-produced. Second, in order to produce high quality color images, the formation of minute, undesirable satellite ink droplets that usually trail an ejected main ink droplet must be avoided. Third, when ink is ejected from one nozzle or when ink refills an ink chamber after ink ejection, cross-talk with adjacent nozzles, from which no ink is ejected, must be avoided. To this end, a back flow of ink in a direction opposite to the direction ink is ejected from a nozzle must be prevented during ink ejection. Fourth, for high-speed printing, a cycle beginning with ink ejection and ending with ink refill in the ink channel must be carried out in as short a period of time as possible. In other words, an inkjet printhead must have a high driving frequency.
The above requirements, however, tend to conflict with one another. Furthermore, the performance of an inkjet printhead is closely associated with and affected by the structure and design of an ink chamber, an ink channel, and a heater, as well as by the type of formation and expansion of bubbles, and the relative size of each component.
In an effort to overcome problems related to the above requirements, various inkjet printheads having different structures have already been suggested. However, while conventional inkjet printheads may satisfy some of the aforementioned requirements, they do not completely provide an improved inkjet printing approach.
FIG. 1 illustrates a cross-sectional view of a conventional bubble type inkjet printhead, schematically illustrating a back-shooting type ink ejector. In a back-shooting type printhead, bubbles grow in a direction opposite to a direction in which ink droplets are ejected.
As shown in FIG. 1, in the back-shooting type printhead, a heater 24 is arranged in the vicinity of a nozzle 22 formed on a nozzle plate 20. The heater 24 is connected to electrodes (not shown) for current application and is protected by a passivation layer 26 made of a predetermined material and formed on the nozzle plate 20. The nozzle plate 20 is formed on a substrate 10, and an ink chamber 12 is formed in the substrate 10 to correspond to the nozzle 22. The ink chamber 12 connected to an ink channel 14 is filled with ink 40. The surface of the passivation layer 26 for passivating the heater 24 is generally coated with an anti-wetting layer 30. The anti-wetting layer 30 prevents the ink 40 from adhering to the passivation layer 26.
In the above-described ink ejector, when current is applied to the heater 24, the heater 24 generates heat and bubbles 44 are produced in the ink 40 filling the ink chamber 12. Thereafter, the bubbles 44 continue to grow by the heat supplied from the heater 24. Accordingly, pressure is applied to the ink 40 so that the ink 40 near the nozzle 22 is ejected through the nozzle 22 in the form of an ink droplet 42. Then, the ink 40 is supplied to the ink chamber 12 through the ink channel 14 and the ink chamber 12 is refilled.
As described above, in order for the above-described bubble-jet type inkjet printhead to exhibit high quality printing, ink must be ejected in a stable manner, i.e., in the form of droplets. The size, form and surface quality of a nozzle are important factors that greatly affect the performance of the conventional bubble-jet type inkjet printhead, including the size of an ink droplet ejected, ejection stability and continuous ejection efficiency. In particular, the quality of a portion of the surface of the printhead near the nozzle greatly affects the ejection stability and continuous ejection efficiency.
Generally, if a nozzle and a portion of a surface of the printhead near the nozzle have an anti-wetting property, ink can be perfectly ejected in the form of an ink droplet. Accordingly, the accuracy in location of recording paper where an ink droplet lands and the uniformity in ink droplet dispersion are improved, thereby improving overall print quality. In addition, after ink ejection, a meniscus formed around the aperture of a nozzle is rapidly stabilized, thus preventing external air from being pulled back into the ink chamber and preventing a surface of the printhead around the nozzle from being contaminated.
Alternatively, if a portion of a surface of the printhead near the nozzle is not subjected to anti-wetting treatment, the portion is susceptible to contamination by ink or a foreign substance. Accordingly, print quality and efficiency may deteriorate. As shown FIG. 2, if the surface of the printhead around a nozzle 62 is not subjected to an anti-wetting treatment, a contact angle xcex8 between an ink droplet 72 and the surface of the printhead is small, so that the ink droplet 72 tends to be easily spread on the surface near the nozzle 62. In this case, a desirably shaped ink droplet, such as the one illustrated in FIG. 1, is not formed, nor is a direction of an ink droplet ejection accurately maintained. Additionally, even after ink droplet ejection, ink may remain on the surface near the nozzle 62. If the surface near the nozzle 62 is stained with ink or a foreign substance, a sheet of recording paper may also be stained with the ink or foreign substance, resulting in poor print quality.
Accordingly, in order to improve the reliability and print quality of an inkjet printhead, it is necessary to subject a surface of a printhead to an anti-wetting treatment. As a coating for the anti-wetting treatment, a metal such as gold (Au), palladium (Pd) or tantalum (Ta) has been typically used. However, such a metal having a contact angle of less than 90xc2x0 cannot be suitably used as a coating for an inkjet printhead that is required to have a high anti-wetting property.
In an effort to solve the above problems, it is a feature of an embodiment of the present invention to provide a bubble-jet type inkjet printhead having a hemispherical ink chamber and an anti-wetting film exhibiting good characteristics while satisfying general requirements of a printhead.
To provide the above feature, the present invention provides a bubble-jet type inkjet printhead including a substrate, in which a manifold for supplying ink, an ink chamber having a substantially hemispherical shape and filled with ink to be ejected, and an ink channel for supplying ink from the manifold to the ink chamber, are incorporated, a nozzle plate, formed on the substrate, having a nozzle, through which ink is ejected, the nozzle formed at a location corresponding to the center of the ink chamber, a heater provided on the nozzle plate and surrounding the nozzle, and electrodes provided on the nozzle plate and electrically connected to the heater to supply pulse current to the heater, wherein an anti-wetting coating including a perfluorinated alkene compound on at least a surface around the nozzle is formed on an exposed surface of the printhead.
Preferably, the perfluorinated alkene compound as an anti-wetting compound is perfluorobutene. Also preferably, the anti-wetting coating is deposited by RF glow discharge and can be removed by O2 plasma.
Since an anti-wetting film having a perfluorinated alkene compound provided on the outer surface around a nozzle has a relatively large contact angle, ink ejection can be made in a more stable manner and more accurately. Thus, the reliability and print quality of the inkjet printhead can be improved.
Additionally, an insulation layer may be preferably formed on the nozzle plate where the heater is formed, and the electrodes are preferably formed on the insulation layer. Further, a passivation layer is preferably formed over the electrodes and the insulation layer, and the anti-wetting coating is preferably formed on the passivation layer.
The manifold may be formed on a bottom side of the substrate, and the ink channel may be formed on a bottom of the ink chamber to be in flow communication with the manifold.
Further, a nozzle guide extending downward in the depth direction of the ink chamber may be formed at an edge of the nozzle.
The heater is preferably annular-shaped, with the electrodes connected to opposite locations of the heater on the diameter thereof. Alternatively, the heater may be formed in the shape of the Greek letter omega and the electrodes are connected to both ends of the heater.